1. Field of the Invention
This invention relates generally to pumps for pumping fluids and more particularly to ejector pumps for pumping fluids.
2. Description of the Prior Art
Ejector pumps are well known devices for pumping fluids. Ejectors utilize motive fluid to pump a second fluid. The motive fluid enters the ejector through a motive inlet which narrows to an inlet nozzle. This nozzle ejects the motive fluid at high velocity into a mixing section. The pumped fluid enters the ejector through a suction inlet which is communicated with the mixing section. The second fluid entering the mixing section from the suction inlet is entrained by the high velocity motive fluid thereby pumping the second fluid. The static pressure of the combined fluids leaving the mixing section may be increased if desired by including a diffuser section downstream of the mixing section. The combined fluids are discharged from the ejector via the diffuser or via the mixing section if no diffuser is provided.
Ejector performance and ejector efficiency are strongly influenced by ejector design and the condition of fluids which enter and leave the ejector. The pressures and mass flow rates achieved for the motive, suction and discharge fluids are measures of ejector performance. The ejector efficiency can be defined in terms of the ratio of mass flow rates for motive and suction fluid flows for fixed motive, suction and discharge pressures. An ejector which can pump at higher mass flow rates from the suction side with less mass flow to the motive side at constant inlet and outlet pressure conditions is said to be more efficient.
Presently, ejector design parameters such as nozzle and mixing section throat diameters are selected and fixed based on assumed average or constant design conditions of fluid at the ejector inlets and outlet. For example, under constant motive inlet conditions an ejector having a particular mixing throat size and a particular inlet nozzle size and configuration will produce the desired design pumping and fluid conditions at the ejector suction and ejector outlet or discharge. In some instances, two stage or three stage ejectors must be utilized to achieve the desired design conditions.
Although ejectors have been found suitable for many types of pumping requirements, serious deficiencies in ejector performance and efficiency result when fluid conditions at the ejector openings fluctuate outside the range of design conditions. In the past, an attempt has been made to reduce these deficiencies by utilizing an ejector which is designed for average fluid conditions somewhere near the middle of the expected fluctuating fluid conditions.
However, if fluid conditions at the ejector inlets and outlet are not the average conditions which were expected when the ejector was selected or designed, the prior art ejectors often will work less efficiently and will not achieve the performance desired. For example, as motive fluid pressure drops the amount of suction mass flow rate and pressure will increase in order to maintain the desired discharge pressure. If the motive fluid pressure drops too low, the ejector will either stop pumping or will generate sputtering discharge flows with oscillating discharge pressure. To assure continued flow in the discharge line, often the ejector must be augmented or backed up with a mechanical gas compressor. Even when the ejector is still providing desired discharge pressures, the energy required to drive the ejector will be increased and the ejector efficiency will be reduced because the inlet and suction pressures and flows are different than the conditions for which the nozzle and mixing throat diameters were selected.
Another deficiency of ejectors has been the large increase in size and cost of ejectors capable of handling fluids at pressures greater than 200-300 psi. For low pressure applications, relatively low strength castings which require minor amounts of machining can be used for ejectors. At high pressures, however, thick, high strength forgings and alloys requiring significant machining must be used. The need for machining results from creating the detailed internal features of the ejector.